The land-surface elevation data has been developed nation-wide as the National Elevation Dataset (NED). The NED data used by the watershed tool is modified to create a grid with greatly exaggerated stream channels to resolve problematic watershed delineations, including watersheds delineated from river confluences and watersheds in large flat sinuous areas (the 'canyonized' stream channels exaggerate the vertical difference of the elevation values in NED for use in determining slope and other watershed characteristics). The NHD ArcView Toolkit watershed tool works only with centerline networks developed in the NHD. It is recommended that 1:24,000-scale NHD be used (as opposed to 1:100,000-scale NHD) for the preprocessing steps described in this document (a single source and scale (1:24,000 USGS topographic quadrangles) for all 3 input data layers is an important aspect of the capacity for these steps to work). In addition, a number of states are developing 1:24,000-scale centerline hydrography data layers that conform to NHD standards. This data layer has been attributed with EPA river reach codes and is now available as the National Hydrography Dataset (NHD).
Data layers corresponding to those used by Massachusetts are in various stages of development for use at the national level.Īt the national level, surface-water hydrography at 1:100,000-scale has been developed with a centerline network from USGS DLG data. Each of these data layers was modified to work in conjunction with the watershed tool and each other. Preparing to use the NHD Watershed tool, as was the case with the Massachusetts tool, is based on modifying 3 spatial datasets: surface-water hydrography, land-surface elevation, and basin boundaries. The processing described in this document enhances the tools developed for Massachusetts by integrating the functionality of the watershed tool in the NHD ArcView Toolkit with spatial data and related databases that cover (or are planned to cover) the continental United States. The tool was developed for watershed analysis, including the WEB application named STREAMSTATS (Ries and others, 2000), using data layers unique to the State of Massachusetts and thus was not immediately transferable to data layers available nationally.
NHD Watershed is an enhancement to a watershed tool that was initially developed for the State of Massachusetts by MassGIS and the U.S.
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This document describes how to collect and preprocess the support data layers.
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